1. Field of Invention
This invention is directed to transducing positional displacements using speckle-image-correlation.
2. Description of Related Art
Various known devices use speckle images and correlation between speckle images to determine deformations and/or displacements. A speckle image is generated by illuminating an optically rough surface with a light source. Generally, the light source is a coherent light source, and more particularly, is a laser-generating light source, such as a laser, a laser diode, and the like. After the optically rough surface is illuminated by the light source, the light scattered from the optically rough surface is imaged onto an optical sensor, such as a charge-coupled device (CCD), a semi-conductor image sensor array, such as a CMOS image sensor array, or the like.
Prior to displacement or deformation of the optically rough sensor, a first speckle image is captured and stored. Subsequently, after deformation or displacement of the optically rough surface, a second speckle image is captured and stored. The previous and subsequent speckle images are then compared on a pixel by pixel basis. In particular, a plurality of comparisons are performed. In each comparison, the previous and subsequent speckle images are offset relative to each other, and the amount of offset is increased by one image element, or pixel, between each comparison. In each comparison, the image value of a particular pixel in the previous image is multiplied by, or subtracted from, the corresponding subsequent image pixel (based on the amount of offset) and the result is accumulated. The offset having the greatest correlation between the subsequent and previous images will generate a peak or a trough when the offset is plotted against the total accumulated value.
For example, U.S. Pat. No. 4,794,384 discloses a mouse which uses speckle-image-correlation to determine two dimensional motion directions of a mouse. In particular, in the 384 patent, the speckle-image-correlation does not need to be performed at a high rate and the accuracy only needs to be on the millimeter range.
U.S. Pat. No. 4,967,093 discloses systems and methods for measuring deformation of an object using speckle-image-correlation. In particular, the 093 patent describes in detail various conventional methods for comparing two speckle images and for determining when to update a current reference speckle image with a new reference speckle image. Similarly, published Japanese Patent Application 8-271231, published October 1996, discloses additional methods for avoiding accumulating error in a speckle-image-based displacement gage.
Finally, published Japanese Patent Application 5-52517, published March 1993, discloses a speckled-image-based displacement meter that uses a rectangular or elliptically shaped slit 51 in a slit plate 5. The light beam from the laser light source passes through the slit 51 before it illuminates the optically rough surface. Thus, the light beam is shaped by the slit 51. The shaped light beam allows an amount of displacement in a main displacement direction to be measured with high sensitivity while displacement components in a perpendicular direction relative to the main displacement direction do not effect the sensitivity of the device.
However, the above-described conventional speckle-image-correlation systems either determine surface displacement of speckle images to analyze body deformations and strain, where it is desirable to maximize the speckle effect of all surface motions, for determining low-resolution motions generated by a computer mouse or other low-resolution measurement devices. In particular, in these conventional speckle-image-correlations systems, there is usually no need to determine, to a high degree of accuracy, the motion of the rigid body along one or more prescribed axes of motion.
In those prior art devices that use speckle-image-correlation in high-accuracy positioning encoders and the like, the practical problems that effectively prevent determining position to a high resolution in a commercially marketable form have not been adequately considered. In particular, these prior art high-accuracy positioning encoders and the like implicitly assume that highly stable structures and highly accurate bearing systems can be used in particular implementations of such speckle-image-correlation, high-accuracy positioning encoders and the like. However, such high-precision mechanical systems are expensive. Furthermore, at the high levels of resolution and accuracy that are commercially demanded in the art, even high-accuracy mechanical systems exhibit unwanted measurement errors, due to play the in bearings, non-planar surfaces, and the like.
This invention provides speckle-image-correlation-based position transducers that enable high-resolution determination of position or displacement.
This invention separately provides speckle-image-correlation-based position transducers that have reduced sensitivity to lateral offsets.
This invention further provides speckle-image-correlation-based position transducers that have reduced sensitivity to lateral offsets by having an aperture which is longer in the direction parallel to the direction of displacement than the aperture is in a direction perpendicular to the direction of displacement.
This invention further provides speckle-image-correlation-based position transducers that have reduced sensitivity to lateral offsets by placing a cylindrical lens between the optically rough surface and a detector.
This invention separately provides speckle-image-correlation-based position transducers that have reduced sensitivity to separations between the optically rough surface and the light source and/or a detector in a direction normal to the optically rough surface.
This invention further provides speckle-image-correlation-based position transducers that are relatively insensitive to relative motions between the optically rough surface and the light source and/or detector in the direction normal to the optically rough surface.
This invention separately provides speckle-image-correlation-based position transducers that are usable to determine displacement for optically rough objects moving at a relatively high velocity.
This invention further provides speckle-image-correlation-based position transducers that strobe the light source to freeze the image during the exposure time of the imaging device to determine displacement for optically rough objects moving at a relatively high velocity.
This invention separately provides speckle-image-correlation based position transducers that have an improved cost/performance ratio.
This invention separately provides speckle-image-correlation based position transducers that have improved robustness and economy.
In various exemplary embodiments of the speckle-image-correlation-based position transducers according to this invention, a light source outputs a light beam towards an optically rough surface. Due to diffraction, the light scattered from the optically rough surface contains a random pattern of bright spots, or speckles. The light scattered from the optically rough surface is imaged onto an image detector having a two-dimensional array of light-sensitive elements. The image captured on the image detector is input and stored. Subsequently, a second image is captured and stored. The two images are then compared on a pixel-by-pixel basis, first without any offsets between the two images in a particular displacement direction. The two images are then compared, each time at a different offset in the particular displacement direction. The comparison having the highest, or lowest, comparison value indicates the amount of displacement of the optically rough surface relative to the light source that occurred between taking the two images.
In particular, in the various exemplary embodiments of the speckle-image-correlation-based position transducers according to this invention, an optical system is placed between the optically rough surface and the image detector. In various exemplary embodiments, the optical system includes a lens and a pinhole aperture. In various exemplary embodiments of the optical system, the pinhole aperture is round and has a diameter chosen so that the average size of the speckles of the random speckle pattern is at least approximately equal to, and in various exemplary embodiment, larger than, the dimensions of the square light-sensitive elements of the image detector.
In other exemplary embodiments of the optical system, the dimension of the pinhole aperture in the direction perpendicular to the direction of displacement is reduced. As a result, the image of the speckles in the direction perpendicular to the direction of displacement is greater than the dimension of the light-sensitive elements of the image detector in that direction. Accordingly, speckle-image-correlation-based position transducers having such pinhole apertures become relatively insensitive to lateral offsets.
In yet other exemplary embodiments of the optical system, the lens is a cylindrical lens that magnifies the relative motions along the direction of displacement but does not magnify relative motions in the direction laterally perpendicular to the direction of displacement. In yet even other exemplary embodiments of the optical system, the optical system is telecentric. As a result, the speckle-image-correlation-based position transducers becomes relatively less sensitive to both separation between the optical system and the optically rough surface, as well as any relative motions between the optical system and the optically rough surface.
In various exemplary embodiments of the speckle-image-correlation-based position transducers according to this invention, the light source is modulated to prevent smearing of the speckle images across the array of light-sensitive elements of the image detector. In particular, in various exemplary embodiments, the light source is strobed for a short period of time to effectively freeze the image during the exposure time of the image detector, which is significantly longer than the strobe period of the light source.
In various exemplary embodiments of the speckle-image-correlation-based position transducer according to this invention, the light source, the optical system, and the image detector are incorporated into a readhead which is moving relative to the optically rough surface, along a one-dimensional displacement axis. In particular, in various exemplary embodiments of the light source, the light source is an optically coherent light source. In particular, in various exemplary embodiments of the coherent light source, the coherent light source is a laser.
In a first exemplary embodiment, the light beam emitted by the light source is emitted at an angle relative to an optical axis of the optical system. The optically rough surface scatters the speckle pattern towards the optical system, which images the speckle pattern on the image detector. In a second exemplary embodiment, the light beam emitted by the light source is emitted at an angle relative to the optical axis of the optical system onto a beamsplitter. That beamsplitter redirects the emitted light beam so that the beam is parallel to the optical axis before the light beam is scattered off the optically rough surface. The optically rough surface scatters the light beam back along the optical axis, through the optical system, including the beamsplitter, and onto the image detector.
In a third exemplary embodiment of the optical system, the light beam is emitted by the light source along the optical axis. In this third exemplary embodiment, the optical system is integrated into a block of material that is optically transparent at the wavelength of the light beam emitted by the light source. In particular, the lens of the optical system is either formed integrally with the block of material or is adhesively attached to block of material. Additionally, in this third exemplary embodiment, rather than a pinhole aperture formed in an otherwise opaque material, the integral optical system includes a semi-transparent thin film that is deposited onto one surface of the optically transparent material across the optical axis.
In particular, this semi-transparent thin film acts as a reverse pinhole, in that the semi-transparent thin film reflects only as much of the scattered light beam as normally is passed by the pinhole aperture. The reverse pinhole semi-transparent thin film redirects the scattered light beam within the block of optically transparent material, to a fully reflective thin-film formed on an opposite side of the block of optically transparent material. The fully reflective thin-film then reflects the speckle image pattern onto the image detector.
These and other features and advantages of this invention are described in or are apparent from the following detailed description of various exemplary embodiments of the systems and methods according to this invention.